The objectives of this research are to discover the proliferating cell nuclear antigen (PCNA) binding site on yeast MutLalpha, a component of the mismatch repair machinery, and to discover if MutLalpha can interact with the 9-1-1 complex, a set of proteins involved in cell cycle checkpoint control. In addition to mismatch repair, MutLalpha is involved in apoptosis signaling, nucleotide excision repair, and meiotic recombination. Mutator phenotypes of variant MutLalpha complexes include cancer and infertility. This research would determine how and if PCNA and/or the 9-1-1 complex are also involved in these processes. These findings would clearly have important implications for cancer and fertility research. To reach the objectives of the project, protein mutagenesis will be used to make pre-determined variant MutLalpha complexes. Both wild-type and variant complexes will be assayed for their interaction with PCNA (and the 9-1-1 complex) in vitro using affinity chromatography and atomic force microscopy and in vivo using a yeast two-hybrid assay. Variants which do not interact with PCNA (or the 9-1-1 complex) will be assayed for their mutator phenotypes.